The N 2 + O + charge‐exchange reaction and the dayglow N 2 + emission

Abstract

Dayglow spectra were recorded by the Arizona Airglow Experiment from the payload bay of the shuttle, STS‐74. These spectra are used to reexamine the role of the prominent N 2 + first negative emission from the dayglow thermosphere. Many reports of the N 2 + emissions identify problems in validating the intensity of the emission. Also, an extended vibrational and rotational structure of the bands remains unexplained in the historical analysis. These anomalies appear to be due to the charge‐exchange reaction, N 2 +O + ( 2 D , 2 P )→N 2 + +O, which is the dominant source of N 2 + ions in the sunlit atmosphere at high altitudes. In the present work the N 2 + emission was considered to originate from two separate ion sources. First are those emissions originating from ions produced by photoionizaton and electron bombardment; these emissions can be modeled. Second are the emissions originating from ions produced by the charge‐exchange reaction; these emissions cannot be modeled. Synthetic emission profiles due to the first ion source were modeled and subtracted from the observed spectrum, leaving emission profiles resulting from the charge‐exchange ion source. These residual vibrational and rotational profiles were analyzed to retrieve resonance scattering rates for these ions. These scattering rates can be used to estimate the N 2 + first negative emission rate expected from the thermosphere with a model of the atmosphere. It is suggested that measurements of the N 2 + emission rate can be used to determine the daytime concentration of the oxygen ion, O + ( 2 D , 2 P ). Although the present work appears to resolve the question of the extended vibrational and rotational band structure, it does not help the excess intensity problem significantly. It does point out that O + must play an important role in intensity problems.